The present invention relates in general to a blown film extrusion apparatus and method, and more particularly, to such an apparatus and method which produces a stabilized, high stalk between spaced-apart tendem air rings for increasing the production rate of blown, low to high molecular weight polymers, while improving the film's physical and mechanical properties.
The use of spaced-apart tandem air rings has been reported to improve the production rate as well as the physical and mechanical properties of blown plastic film. These improvements are attributed in part to the creation of a high stalk of a semi-molten state which allows the viscoelastic forces of the extruded film to dissipate before expansion. This artificially created condition produces improved drawdown characteristics and allows the molecular re-orientation of the film to provide increased impact strength. However, although an improvement over conventional blown film, the tandem air ring arrangement results in limited film width and production rate capabilities due to instability of the high stalk, i.e., oscillation of the film about its longitudinal axis whereby undesirable wrinkles form in the finally collapsed film. In addition to the formation of wrinkles, severe instability of the high stalk at increased production rates can result in tearing of the blown film which cause its ultimate collapse during the extrusion processes.